Pisciculture is the breeding and culture of fish in fresh water or salt water tanks which define evolutionary volumes for a family of fish induced to develop and to grow by a daily controlled supply of food.
The growth of such fish involves controlling their size, either in order to transfer them periodically from the tanks in which a force and/or weight ratio can be established and must be maintained, or in order to comply with the requirements of their commercialization.
It is therefore necessary to be able to grade the fish issued from a tank, so as to determine their growth rate, but also in order to be able to sort them according to weight or length criteria, and thus to divide them into different predetermined categories, either to put them in tanks where they will continue to grow, or to meet subsequent commercialization requirements.
Machines for carrying out this type of work are already known and used. Most of them propose to delimit at least one grading channel, of which the open bottom section increases gradually from an inlet end towards an outlet end, in order to permit gradual selection as a function of the size of the fish admitted to go through such a channel, when the local passage section corresponds to their larger cross-section.
The fish having gone through the open bottom of the grading channel are received in transfer cases or compartments.
For the grading, the prior art proposes several embodiments of grading channel.
One solution consists in delimiting the grading channel by two flat panels, the relative spacing of which is adjustable, said panels reaching to above two substantially parallel rollers. Said rollers, of which the relative spacing is also adjustable, are driven in reverse rotation, so that a fish picked up locally between them, has a tendency to be lifted and rejected toward the top of the channel.
Normally in such a channel, the fish are held in vertical engagement, the tail facing downwardly. The reverse rotation of the rollers is selected in the direction indicated hereinabove in order to prevent the fish from being drawn in-between the rollers, which would cause them to be compressed and harm them.
In actual fact, such a machine is merely an adaptation of a machine initially designed for grading fruit. The design of that machine is ill-suited for grading live fish which are not adequately guided and carried by the rotating rollers. Therefore a suitable grading cannot be obtained.
Another known prior art machine proposes to define a grading channel, not with two rigid panels, but with two endless belts, the spacing of which can be conveniently adjusted.
The belts are driven forward, in such a way that, by their ends in facing relationship, they can push the live fish forward from the entrance end toward the exit end of the channel having the largest passage section of the open bottom.
The working principle of this particular embodiment, can be deemed to ensure, theoretically, a better guiding of the live fish. However, the driving belts are flexible members, locally subjected to vibrations or flapping movements which are not always synchronous and in-tune. Such flapping movements are responsible for significant variations of the passage section of the open bottom that they define. Therefore, the grading results are, in this case also, doubtful.
Experience has also shown that large-sized fish could bear traces of chaffing, because of the instability imposed on them by the longitudinal movement of the endless belts. And fish, which have been marked in this way, are, later on, subject to physical degradations which are detrimental to them.
The prior art further proposes another machine in which the grading channel is constituted by two panels defining an open-bottomed V-shaped channel. In such a channel, a pressing belt is driven endlessly forward longitudinally. Such a belt is provided with flexible fingers which are engaged in the channel, in such a way as to cause the fish to advance in a horizontal position until such time as the passage section of the open bottom allows them to pass through the channel.
Such a machine is satisfactory form the standpoint of the function to be carried out, but it presents certain drawbacks, due mainly to its rather complex structure. The production and maintenance costs of such a machine are higher than those of the previously described machines. Such a machine demands accurate maintenance, especially as regards the tension of the endless belts, so that the flexible fingers are suspended at the correct height and are driven forward at a speed compatible with the fish dwelling time inside the grading channel.
To sum up, therefore, the prior technique does not offer any solutions which can be regarded as completely satisfactory to carry out the grading and sorting of oblong-bodied live fish, such as fish of the Salmonidae family.